1. Field of the Invention (Technical Field)
This invention relates to navigation systems, and, in particular, airborne navigation and guidance systems utilizing the computation of position and velocity for automatic guidance.
2. Background Art
Modern navigation systems utilize extremely accurate signals from a constellation of satellites that comprise the Global Positioning System (GPS) to compute position. However, while such signals provide excellent long-term position accuracy, they are noisy in the short-term. This is particularly true of the velocity data provided. Velocity data as well as position data is required to provide smooth, stable guidance. A further disadvantage of GPS is that the data rate of the position solution is lower than desirable for smooth, stable aircraft guidance.
Current practice is to augment the GPS signals with signals from Inertial Navigation Systems (INS). Present day commercial systems compute the difference between unfiltered INS position, and unfiltered GPS position, and then filter the correction signal in a second order lag (not complementary) filter before applying it to the INS. The computational complexity of such approaches is equivalent to the instant invention, but provides none of the advantages. The INS signals exhibit excellent short-term accuracy and smoothness, but they suffer from long-term drift that produces position errors that are unbounded with time. By blending the GPS and INS signals, a blend can be produced that exhibits the best characteristics of each. Not only is accurate position data produced, but also, velocity data that is smooth. Further, the velocity data can be used to xe2x80x9cfillxe2x80x9d the position data between GPS position updates. The accurate position data can be used to correct the INS data from long term-drift as well.
A common, albeit complicated, method to accomplish the blending of GPS and INS data that also corrects the INS long term drift is the Kalman Filter. Such a filter can correct both drift and drift rate of an INS, but requires a very powerful digital computer to provide acceptable data rates for smooth, stable guidance.
The current invention provides both smooth long-term and short-term signals whose quality and data rate are high enough to be used for guidance, and provides signals to correct INS drift and drift rate. However, it is simple and straightforward to implement, and does not require a powerful computer in order to provide the necessary functionality and update rate for guidance. In fact, it could be implemented in an analog computer.
The two means of solving the problem are Kalman Filters, and xe2x80x9clagxe2x80x9d filters. The former are used commonly in Military systems, (for instance the C-141C GPS Enhanced Navigation System or GPSENS) while the latter are used in Commercial systems such as Honeywell""s Flight Management System for the Boeing 757, 767, and 777 aircraft.
The Kalman filter is the standard solution to the problem of providing blended GPS and INS data for use in guidance systems while also providing correction signals for the INS drift and drift rate. However, it requires a powerful computer to produce acceptable data rates, or its output must be augmented in other ways to be acceptable for use in guidance systems. This prior art solution is very costly and complex. Kalman filters may solve the problem of correcting the INS data for drift and drift rate, but that is not necessarily their primary purpose. Kalman filters combine signals from a multiplicity of sources, in an effort to produce an output that is statistically more accurate than any of the inputs.
Commercial systems use lag filters to eliminate noise from the GPS position information so that it can be used to correct the INS information that is used for guidance. However, such filters cannot provide correction for INS drift rate, and may limit the effectiveness of the guidance system in which they are used.
Prior art is concerned with using a simple second order filter (Not complementary) to determine the magnitude, velocity, or acceleration of the error between two sensor signals (1), or optimizing the blending of signals at the xe2x80x9crawxe2x80x9d measurement level, using a plurality of measurements to determine the integrity of any single measurement in a conventional (2-5) or Kalman filter Kalman (6). None of the prior art used position from GPS and both position and velocity from INS to correct errors in both the position and velocity output of an INS as used in the present invention.
Other devices are similar in that they utilize both velocity and position data in combination to provide smooth output signal. However, such filters to not use the velocity error that may be available in such devices to correct for drift rate of the INS. For an example, see the Radio/INS filter that is used in the C-141C Navigation subsystem as shown in FIG. 1.
The current invention improves on the prior art in that it inherently compensates for errors in the velocity input as it uses the velocity data in the blended output. It therefore corrects both position and velocity errors using signal from a single filter.
In accordance with the present invention, disclosed is a second order complementary filter for correcting position errors and velocity errors in an inertial navigation system (INS). The preferred second order complementary filter for correcting position errors and velocity errors in an inertial navigation system comprises a blending filter for blending a position signal from a first navigation source with a velocity signal from a second navigation source, structure for generating an output from the blending filter comprising a rate of change of position error of the first navigation source from the second navigation source, an integrator for inputting the rate of change of position error when the first navigation source becomes invalid and outputting a dynamic position error signal and an apparatus for summing the dynamic position error signal and an inertial navigation system position signal to output a dynamically corrected inertial navigation system position. The first navigation source preferably comprises a global positioning system and the second navigation source comprises the INS. The preferred integrator comprises an INS correction loop for continuous compensation for an INS drift rate when the first navigation source becomes invalid. The preferred invention further comprises a switch for rendering the correction loop operational when the first navigation source becomes invalid. The preferred switch further comprises a hold position for making the output of the integrator constant.
The preferred method of correcting position and velocity errors with a second order complementary filter in an inertial navigation system comprises the steps of blending a position signal from a first navigation source with a velocity signal from a second navigation source with blending filter, generating an output from the blending filter comprising a rate of change of position error of the first navigation source from the second navigation source, creating a position correction signal from the generated output and applying the corrected signal to the inertial navigation system. The preferred method further comprises the step of continuously repeating the aforementioned steps. The preferred method further comprises the step of capturing a last value of position error when the first navigation source becomes invalid. The preferred method further comprises the step of switching the second order complimentary filter on when the first navigation source becomes invalid. The preferred step of switching further comprises placing the second order complimentary filter on hold so that the generated output is constant.
An alternative apparatus comprises a second order complementary filter for correcting position errors and velocity errors in an inertial navigation system (INS) comprising a blending filter for blending a position signal from a first navigation source with an inertial navigation velocity signal from a second navigation source, structure for generating a dynamic error signal from the blending filter comprising a rate of change of position error of the first navigation source from the second navigation source, an apparatus for disconnecting a position input and a position feedback from the blending filter when the input position signal becomes invalid, an apparatus for summing the dynamic error signal and the inertial navigation system velocity signal to produce a dynamically corrected inertial navigation system velocity signal and an apparatus for integrating the inertial navigation velocity signal to produce a corrected inertial navigation system position signal. The first navigation source preferably comprises a global positioning system and the second navigation source comprises the INS. The apparatus for integrating is preferably part of the second order complementary filter.
A primary object of the present invention is to blend GPS and INS signals for correction of drift and drift rate errors.
Another object of the present invention is to provide a second order complementary filter to determine drift rate of an INS, and the use of this drift rate to improve the correction to the position error of the INS.
Another object of the present invention is to provide a second order complementary filter or correct an INS velocity signal so that this corrected velocity signal can be used to compute accurate position information in the presence of momentary interruption of GPS.
Another object of the present invention is to provide smoothed position and velocity information of suitable dynamic fidelity to be used in a stable. accurate guidance system in an aircraft.
A primary advantage of the present invention is that it is simple and inexpensive compared to prior art methods which are complicated and expensive and do not satisfy all the needs that the present invention satisfies.
Other objects, advantages and novel features, and further scope of applicability of the present invention will be set forth in part in the detailed description to follow, taken in conjunction with the accompanying drawings, and in part will become apparent to those skilled in the art upon examination of the following, or may be learned by practice of the invention. The objects and advantages of the invention may be realized and attained by means of the instrumentalities and combinations particularly pointed out in the appended claims.